the industrial applicability of pea space charge
TRANSCRIPT
energies
Article
The Industrial Applicability of PEA Space ChargeMeasurements, for Performance Optimization ofHVDC Power Cables
Antonino Imburgia 1 , Pietro Romano 1,* , George Chen 2, Giuseppe Rizzo 1 ,Eleonora Riva Sanseverino 1, Fabio Viola 1 and Guido Ala 1
1 LEPRE Laboratory, Dipartimento di Ingegneria, Università di Palermo, 90128 Palermo, Italy;[email protected] (A.I.); [email protected] (G.R.);[email protected] (E.R.S.); [email protected] (F.V.); [email protected] (G.A.)
2 The Tony Davies High Voltage Laboratory, University of Southampton, Southampton SO17 IBJ, UK;[email protected]
* Correspondence: [email protected]
Received: 12 September 2019; Accepted: 29 October 2019; Published: 2 November 2019�����������������
Abstract: Cable manufacturing industries are constantly trying to improve the electrical performanceof power cables. During the years, it was found that one of the most relevant degradation factorsinfluencing the cable lifetime is the presence of space charge in the insulation layer. To detect theaccumulated charge, the pulsed electro-acoustic (PEA) method is the most used technique. Despitethe wide use of the PEA cell, several issues are still present. In particular, the PEA output signal isstrongly disturbed by the acoustic waves reflections within the PEA cell. This causes the distortion ofthe output signal and therefore the misinterpretation of the charge profiles. This, in turn, may result inan incorrect cable characterization from the space charge phenomenon point of view. In 2017, due tothe proved degradation effect of the space charge accumulation phenomenon, the IEEE Std 1732 wasdeveloped. This standard describes the steps to be followed for the space charge measurement incables specimens during pre-qualification or type tests. Therefore, cable manufacturing industriesstarted to take a particular interest in these measures. In the light of this, the aim of the presentwork is to highlight that the enacted standard is not easily applicable since various problems arestill present in the PEA method for cables. In particular, in this work, the effect of multiple reflectedsignals due to the different interfaces involved, but also the effect of the signal attenuation due tocable dielectric thickness, as well as the effect of the PEA cell ground electrode thickness in the outputcharge profile, are reported. These issues have been demonstrated by means of an experimental testcarried out on a full-size cable in the Prysmian Group High Voltage laboratory. To better understandthe PEA cell output signal formation, a PEA cell model was developed in a previous work and ithas been experimentally validated here. In particular, simulations have been useful to highlight theeffect of the reflection phenomena due to the PEA cell ground electrode thickness on the basis ofthe specimen under test features. Moreover, by analyzing the simulation results, it was possibleto separate the main signal from the reflected waves and, in turn, to calculate the suitable groundelectrode thickness for the cable specimen under test.
Keywords: space charge; PEA method; PEA cell model; IEEE Std 1732; space charge in cables
1. Introduction
Energy efficiency in both high voltage alternating current (HVAC) and high voltage direct current(HVDC) transmission systems strictly depends on the cable design. One of the most important elements
Energies 2019, 12, 4186; doi:10.3390/en12214186 www.mdpi.com/journal/energies
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in a cable is the insulation layer [1–3]. For this reason, cable manufacturers are continuously testingdielectric materials that can provide the best performances [4,5].
Typically, from the electrical point of view, capacitance, resistivity, dielectric strength,and permittivity are the main parameters to be taken into account [6]. Cross linked polyethylene(XLPE) material was found to be the best solution for these systems, as it has a high dielectric strength,low dielectric permittivity, and low electrical resistivity compared to other dielectric materials [7].Based on its satisfactory characteristics for HVAC applications [8], in the last decades this dielectricmaterial has also been used in HVDC systems [9–12]. Unfortunately, due to the physical and chemicalstructure of XLPE material, under a constant DC stress, the space charge accumulation phenomenon isstrongly present [13–16].
The presence of space charge will result in a local increment of the electric field within theinsulating layer, which causes the reduction of its electrical performance and lifetime [17]. Additionally,the presence of space charges may change the insulating properties of the material itself as well as theleakage current value. This translates in a different energy efficiency of the cable and therefore of theentire HVDC system.
Based on the considerations above, in 2017, some experts in the field of high voltage testssuggested the introduction of the space charge measurement for cables, during pre-qualification ortype tests load cycle. Therefore, according to “IEEE Std 1732-2017” standard, both thermal step method(TSM) [18,19] or the pulsed electro-acoustic (PEA) [20] technique can be used to test the full-size cablesspecimens. Both measurement techniques are widely described and compared in [21–23]. In theproposed work, the PEA method has been analyzed and used for the experimental test. Its workingprinciple is essentially based on the generation, propagation, and detection of acoustic waves whichcarry information on the accumulated space charges. Despite the wide use of the PEA method, severalissues are still present. In particular, the presence of acoustic wave reflections, due to the differentinterfaces involved in both PEA cell and samples under test, generates the distortion of the PEA outputsignal which could result in a misinterpretation of the real accumulated space charge [24,25]. Typically,in the PEA cell output signal only two main peaks due to positive and negative surface chargesaccumulated in the dielectric interfaces should be present. However, if the reflected waves fall betweenthe two main peaks a signal distortion occurs [26]. For this reason, it is important to evaluate before orafter a space charge measurement the acoustic wave behavior with the aim to establish the temporallocation of both main peaks as well as their reflections. For this aim, in a previous work, a PEA cellmodel for flat specimens has been developed by the authors [27,28]. In this work, after minor changes,the same model has been used for a cable specimen. The changes essentially concern the introductionof the three layers specimen representing the cable (one dielectric and two semiconductors layers),as well as the different PEA cell component dimensions.
In this way, the spurious signals can be separated from the original one and thus the evaluation ofthe accumulated charges becomes more accurate. Thanks to this study, the output signal interpretationerrors are overcome and thus it is possible to choose the best cable type, as well as the best dielectricmaterial. The latter should be the one that shows the best behavior from the space charge accumulationpoint of view.
In the first part of this article, a brief description of the space charge accumulation phenomenonis given. Then, the standard IEEE 1732-2017 has been summarized in ten main points. After that,the PEA cell for cable specimens and its features are described. In paragraph VI, an experimentaltest in a full-size cable carried out in the high voltage laboratory of a cable manufacturing industry,is reported in detail. The issues found in the obtained PEA cell output signal are thoroughly discussedand reproduced with the simulation model. Finally, a theoretical solution is given, as well as twographs useful to facilitate and make faster selection of the ground electrode and cable features in orderto avoid reflections in the main space charge profile.
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2. The Space Charge Accumulation Phenomenon
The space charge accumulation phenomenon, which means trapped electrons or ions in thematerial bulk, occurs in the insulating layer of cables, joints, and terminations employed in HVDCtransmission [7]. This phenomenon is influenced by several factors, such as the voltage level,the duration of the applied voltage, the contact between dielectric material and semiconductor layer,as well as the chemical and physical properties of the dielectric material. Over that, insulation physicaland geometrical parameters as well as transient conditions have a fundamental role in the space chargeaccumulation [29]. The main effect of this phenomenon is the distortion of the original Laplacianelectric field distribution, which could cause extremely high local electric fields. This in turn may causethe insulating material to degrade and this fact can lead in turn to electrical breakdown [30,31].
To better explain this phenomenon, in Figure 1 a HVDC cable with accumulated space chargesis illustrated. Considering the cable subjected to a positive high voltage, in the initial stage onlythe surface charges are present. In particular, the positive charges (in red color) are accumulated incorrespondence of the inner semicon/dielectric interface, while, the negative surface charge (blue color)in the dielectric/outer semicon interface. After a certain time, the accumulation of charges occurs alsoin the insulation bulk. These charges can be injected from the electrodes (the core of the cable) orgenerated in the dielectric itself due, for example, to physical or chemical defects [7].
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transmission [7]. This phenomenon is influenced by several factors, such as the voltage level, the duration of the applied voltage, the contact between dielectric material and semiconductor layer, as well as the chemical and physical properties of the dielectric material. Over that, insulation physical and geometrical parameters as well as transient conditions have a fundamental role in the space charge accumulation [29]. The main effect of this phenomenon is the distortion of the original Laplacian electric field distribution, which could cause extremely high local electric fields. This in turn may cause the insulating material to degrade and this fact can lead in turn to electrical breakdown [30,31].
To better explain this phenomenon, in Figure 1 a HVDC cable with accumulated space charges is illustrated. Considering the cable subjected to a positive high voltage, in the initial stage only the surface charges are present. In particular, the positive charges (in red color) are accumulated in correspondence of the inner semicon/dielectric interface, while, the negative surface charge (blue color) in the dielectric/outer semicon interface. After a certain time, the accumulation of charges occurs also in the insulation bulk. These charges can be injected from the electrodes (the core of the cable) or generated in the dielectric itself due, for example, to physical or chemical defects [7].
With reference to the cable in Figure 1, the typical space charge distributions measured with the PEA method and the related electric field profiles are reported in Figure 2a,b, respectively. These patterns, obtained with the simulation software, show the charges and the electric field behaviors at 5 min, 12 and 24 h after voltage application.
Figure 1. High voltage direct current (HVDC) cable with accumulated surface and space charges.
(a) (b)
Figure 2. (a) Space charge distribution in a full-size cable and (b) related electric field profiles.
3. The Standard IEEE 1732-2017
Based on the space charge degradation effect, to improve the cables performance and to increase their lifetime, in 2017, a recommendation named “Recommended Practice for Space Charge Measurements on High-Voltage Direct-Current Extruded Cables for Rated Voltages up to 550 kV” was proposed [32] by IEEE. In this standard, the authors suggest a protocol to be followed for the
Figure 1. High voltage direct current (HVDC) cable with accumulated surface and space charges.
With reference to the cable in Figure 1, the typical space charge distributions measured with the PEAmethod and the related electric field profiles are reported in Figure 2a,b, respectively. These patterns,obtained with the simulation software, show the charges and the electric field behaviors at 5 min,12 and 24 h after voltage application.
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transmission [7]. This phenomenon is influenced by several factors, such as the voltage level, the duration of the applied voltage, the contact between dielectric material and semiconductor layer, as well as the chemical and physical properties of the dielectric material. Over that, insulation physical and geometrical parameters as well as transient conditions have a fundamental role in the space charge accumulation [29]. The main effect of this phenomenon is the distortion of the original Laplacian electric field distribution, which could cause extremely high local electric fields. This in turn may cause the insulating material to degrade and this fact can lead in turn to electrical breakdown [30,31].
To better explain this phenomenon, in Figure 1 a HVDC cable with accumulated space charges is illustrated. Considering the cable subjected to a positive high voltage, in the initial stage only the surface charges are present. In particular, the positive charges (in red color) are accumulated in correspondence of the inner semicon/dielectric interface, while, the negative surface charge (blue color) in the dielectric/outer semicon interface. After a certain time, the accumulation of charges occurs also in the insulation bulk. These charges can be injected from the electrodes (the core of the cable) or generated in the dielectric itself due, for example, to physical or chemical defects [7].
With reference to the cable in Figure 1, the typical space charge distributions measured with the PEA method and the related electric field profiles are reported in Figure 2a,b, respectively. These patterns, obtained with the simulation software, show the charges and the electric field behaviors at 5 min, 12 and 24 h after voltage application.
Figure 1. High voltage direct current (HVDC) cable with accumulated surface and space charges.
(a) (b)
Figure 2. (a) Space charge distribution in a full-size cable and (b) related electric field profiles.
3. The Standard IEEE 1732-2017
Based on the space charge degradation effect, to improve the cables performance and to increase their lifetime, in 2017, a recommendation named “Recommended Practice for Space Charge Measurements on High-Voltage Direct-Current Extruded Cables for Rated Voltages up to 550 kV” was proposed [32] by IEEE. In this standard, the authors suggest a protocol to be followed for the
Figure 2. (a) Space charge distribution in a full-size cable and (b) related electric field profiles.
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3. The Standard IEEE 1732-2017
Based on the space charge degradation effect, to improve the cables performance and toincrease their lifetime, in 2017, a recommendation named “Recommended Practice for Space ChargeMeasurements on High-Voltage Direct-Current Extruded Cables for Rated Voltages up to 550 kV” wasproposed [32] by IEEE. In this standard, the authors suggest a protocol to be followed for the spacecharge measurements during pre-qualification or type test load cycles in cables. For the PEA method,the proposed test procedure can be summarized as follows:
a. Cable heating. Specifically, the same temperature gradient at which the cable is designed tooperate shall be created starting from 24 h before the measurement and maintained during theentire test;
b. Positive voltage pooling time. Now the space charge measurement can start by applying apositive polarity rated voltage;
c. First space charge measurement. After a few minutes of voltage application, the space chargedistribution shall be acquired. This profile is constituted by only surface charges and is used asthe reference;
d. Subsequent measurements. After the “first measurement” and every 60 min until a time equal to3 h a first series of space charge measurements shall be performed;
e. Electric field evaluation. By taking into account the acquired space charge distributions duringthe “first measurement” and after 3 h of voltage application, the related electric field profiles(at 0 and 3 h) are determined. After that, the maximum absolute percentage variation betweenthe two electric field profiles, ∆E, is calculated. If ∆E < 10% the electric field is considered stableand the subsequent step f can be neglected (go directly to point g). Otherwise, other space chargemeasurements are needed;
f. In case of ∆E > 10%. The space charge measurements continue and they are acquired after 4, 5,and 6 h of voltage application. From the last measurement (at 6 h) the electric field distributionis determined and compared with that previously calculated after 3 h. As in the point e themaximum absolute percentage variation between the two electric field profiles is evaluated.Additionally, in this case, if ∆E < 10% go to point g, while, if ∆E is again greater than 10% repeatthe space charge measurements for other 3 h. Therefore, after 7, 8, and 9 h of voltage application.Here, the maximum absolute percentage variation is calculated between the electric field at6 and 9 h;
g. Voltage removal. The cable conductor shall be short-circuited and grounded within a time nolonger than 5 min from the voltage removal;
h. Space charge measurement during volt-off. In this phase, the charge profiles are acquiredimmediately after the beginning of the depolarization time and then after 1, 2, and 3 h. For allacquisitions, the electric field profiles are also plotted, but only the relevant distributions aretaken into account;
i. The cable shall be left grounded for at least 24 h;j. Negative voltage pooling time. The space charge profiles, and the related electric field
distributions are measured by applying a negative polarity rated voltage. The steps fromc to h shall be repeated.
4. The PEA Method for Cables
The PEA measurement setup for cable specimen is shown in Figure 3. It is mainly composedof a cable specimen, a high voltage generator HVDC, a pulse generator ep(t), an oscilloscope, and acomputer. First of all, the high voltage is applied in order to promote the accumulation of space chargesin the cable insulation bulk. Then, to measure the accumulated charges the pulse generator is needed.The pulse voltage is used to vibrate the charges and, from its vibration, acoustic waves are generated.These waves travel to the piezoelectric sensor (which is placed underneath the ground electrode of the
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PEA cell) and are detected. The sensor converts the pressure signal into a voltage signal proportionalto the accumulated charges. Finally, the signal is sent to the oscilloscope and to the computer in orderto be displayed and elaborated. Inside the PEA cell, also an absorber and an amplifier are present.The first component allows the absorption of the acoustic waves with the aim to avoid reflection of thesignal across the piezoelectric sensor. The amplifier, instead, is used to increase the voltage level of thepiezoelectric sensor output signal [20–23].Energies 2019, 12, x FOR PEER REVIEW 5 of 12
Figure 3. Pulsed electro-acoustic (PEA) measurement setup for space charge measurement in cables.
5. Experimental Test on a Cable Specimen
As previously reported, according to the IEEE Std 1732-2017, cable manufacturing industries need to measure the space charge before the cable installation.
Based on the above, a space charge measurement by means of the PEA method has been made in a full-size cable. The 20 m long cable under test, equipped with a 20 mm thick XLPE dielectric layer, has been subjected to a voltage of 380 kV. After applying the pulse voltage with magnitude 8 kV, the PEA cell output signal displayed by the oscilloscope is shown in Figure 4 [33].
Figure 4. PEA cell output signal depicted by the oscilloscope.
As it can be seen, the obtained space charge profile is not very clear. By visual inspection, it is possible to say that the signals named 𝑒 (𝑡) and 𝑒 (𝑡) are the voltage pulse and its reflection in the PEA cell, respectively. The first peak 𝑝 (𝑡) is due to the accumulated charges at the dielectric/external semiconducting layer interface (see the blue charges in Figure 1). While, the signal 𝑝 (𝑡), corresponding to the positive charges (red charges in Figure 1) accumulated at the internal semiconducting layer/dielectric interface, is overlapped with the wave named 𝑝 (𝑡). After the acoustic wave behaviour analysis carried out by means of a simulation model (that will be described in the next section) it has been found that the signal 𝑝 (𝑡) is the reflection of the wave 𝑝 (𝑡) within the ground electrode of the PEA cell.
Figure 3. Pulsed electro-acoustic (PEA) measurement setup for space charge measurement in cables.
5. Experimental Test on a Cable Specimen
As previously reported, according to the IEEE Std 1732-2017, cable manufacturing industries needto measure the space charge before the cable installation.
Based on the above, a space charge measurement by means of the PEA method has been madein a full-size cable. The 20 m long cable under test, equipped with a 20 mm thick XLPE dielectriclayer, has been subjected to a voltage of 380 kV. After applying the pulse voltage with magnitude 8 kV,the PEA cell output signal displayed by the oscilloscope is shown in Figure 4 [33].
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Figure 3. Pulsed electro-acoustic (PEA) measurement setup for space charge measurement in cables.
5. Experimental Test on a Cable Specimen
As previously reported, according to the IEEE Std 1732-2017, cable manufacturing industries need to measure the space charge before the cable installation.
Based on the above, a space charge measurement by means of the PEA method has been made in a full-size cable. The 20 m long cable under test, equipped with a 20 mm thick XLPE dielectric layer, has been subjected to a voltage of 380 kV. After applying the pulse voltage with magnitude 8 kV, the PEA cell output signal displayed by the oscilloscope is shown in Figure 4 [33].
Figure 4. PEA cell output signal depicted by the oscilloscope.
As it can be seen, the obtained space charge profile is not very clear. By visual inspection, it is possible to say that the signals named 𝑒 (𝑡) and 𝑒 (𝑡) are the voltage pulse and its reflection in the PEA cell, respectively. The first peak 𝑝 (𝑡) is due to the accumulated charges at the dielectric/external semiconducting layer interface (see the blue charges in Figure 1). While, the signal 𝑝 (𝑡), corresponding to the positive charges (red charges in Figure 1) accumulated at the internal semiconducting layer/dielectric interface, is overlapped with the wave named 𝑝 (𝑡). After the acoustic wave behaviour analysis carried out by means of a simulation model (that will be described in the next section) it has been found that the signal 𝑝 (𝑡) is the reflection of the wave 𝑝 (𝑡) within the ground electrode of the PEA cell.
Figure 4. PEA cell output signal depicted by the oscilloscope.
As it can be seen, the obtained space charge profile is not very clear. By visual inspection, it ispossible to say that the signals named ep(t) and eR
p (t) are the voltage pulse and its reflection in the
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PEA cell, respectively. The first peak p−(t) is due to the accumulated charges at the dielectric/externalsemiconducting layer interface (see the blue charges in Figure 1). While, the signal p+(t), correspondingto the positive charges (red charges in Figure 1) accumulated at the internal semiconductinglayer/dielectric interface, is overlapped with the wave named pR
−(t). After the acoustic wave behaviour
analysis carried out by means of a simulation model (that will be described in the next section) it hasbeen found that the signal pR
−(t) is the reflection of the wave p−(t) within the ground electrode of the
PEA cell.In addition, the reflected signal eR
p (t) falls in the same time range of the main signal and thereforeit is not possible to gain a clear visualization of the accumulated charges in the insulation bulk.
For these reasons, it was not possible to appreciate the space charge profiles and therefore theelectric field distributions, as requested by the IEEE Std 1732-2017.
6. Main Issues of the PEA Method for Cable Specimens
The measurement setup of the PEA method for cable specimens has been reported in Figure 3.As it can be seen, for the application of the pulse generator electrodes some parts of the cable externalmetallic shield must be removed in order to apply the voltage pulse in correspondence of the outersemiconductor layer. This aspect can be seen as the main application problem of the PEA method,because it turns to be destructive for the cable under test.
In addition, during the experimental test reported in the previous Section 5, several other issuesaffecting the PEA cell output signal have been found. In particular:
• The acoustic waves generated in the internal semiconductor/dielectric interface, before reaching thePEA cell, travel within the entire insulation layer. The latter, has both high thickness value (~20 mm)and high acoustic attenuation coefficient. Therefore, the information about the accumulatedcharge in this interface is characterized by a weak signal which, in some cases, could be confusedwith the measurement noise.
• the generated acoustic waves when travelling within both the cable specimen and the PEA cellare subjected to the reflection phenomenon. This is due to the different interfaces involved duringthe waves propagation. Specifically, when an acoustic wave reaches an interface, based on theacoustic impedance of the two materials in touch (e.g., dielectric and aluminum of the groundelectrode), a part of the wave is transmitted and another part is reflected in the opposite direction.The reflected wave, after a certain time, is reflected again and directed towards the piezoelectricsensor. Based on this, the sensor detects both the main signal and its reflections. The latter signals,at least in our experimental test, fall within the main charge peaks and thus are a disturbancewhich do not allow the correct estimation of the accumulated charge.
The above reported issues, and in particular those regarding the reflections phenomena, have beenidentified by means of a PEA cell simulation model, whose working principle and related simulationresults will be described in the next section.
7. The PEA Cell Model
In order to correctly evaluate the signal depicted in Figure 4, a model able to simulate the acousticwaves behavior within the PEA cell and the cable has been used. The model has been developed andwidely described in a previous work published by the authors in [27,28]. The model is essentially basedon the analogy between acoustic and mechanical quantities, such as the analogy between force andvelocity of an acoustic wave with voltage and current of an electrical wave, respectively [34]. Therefore,the Telegraphist’s equations, describing the current and voltage behavior in electrical transmissionlines, have been used to simulate the acoustic wave behavior in the PEA setup. In particular, it waspossible to model each PEA cell component, as well as each cable layer, with a lossy transmissionline modeled as a cascade of two-ports modules. The electrical parameters (resistance, inductance,capacitance, and conductance) of each transmission line have been calculated by means of mechanical
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quantities, such as the material density, the sound velocity, and the attenuation coefficient of the relatedPEA cell setup component. These mechanical quantities as well as the thickness of each PEA cell setupcomponent are required by the model as input data. In addition, the model needs the magnitudevalue of the pulse voltage generated which simulate the generated acoustic waves due to the chargesvibration. Since the model is able to simulate only the acoustic waves behavior and not the accumulatedcharge, the magnitude of the generated voltage pulse Vp, given by Vp = σE (where σ is the dielectricconductibility and E is the electric field within the insulating material) must be calculated before thesimulation and inserted as input data.
By changing the input data (thickness, sound velocity, material density, and attenuation coefficientof the cable layers) the PEA cell model is able to simulate the acoustic wave behavior for differenttypes of cable specimens. However, for different measurement conditions, e.g., temperature gradient,humidity, pressure or mechanical stress, further model developments are needed.
A detailed explanation of the developed PEA cell model used in this work is provided in [27,28].Due to the one-dimensional approach referred to a single charge propagation phenomenon, the
wave propagation model, developed for the flat specimens, has been identically applied to a cylindricalcable geometry. Therefore, despite the model that was developed for a PEA cell testing flat specimen,it works correctly also when the specimen has cylindrical geometry, just as the cable. This is becausethe presence of surface charges has been considered and, in particular, only those with acoustic wavespropagating in the perpendicular direction.
In Figure 5, the configuration implemented into the model is schematically represented. It showsthe section of the full-size cable and some parts of the PEA cell (of course, the entire PEA cell, as thatof Figure 3, has been implemented into the model). In the same figure, the path of the waves p−(t),p+(t), and pR
−(t) are also depicted. The detailed explanation of these acoustic waves course is given in
the following.
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By changing the input data (thickness, sound velocity, material density, and attenuation coefficient of the cable layers) the PEA cell model is able to simulate the acoustic wave behavior for different types of cable specimens. However, for different measurement conditions, e.g., temperature gradient, humidity, pressure or mechanical stress, further model developments are needed.
A detailed explanation of the developed PEA cell model used in this work is provided in [27,28]. Due to the one-dimensional approach referred to a single charge propagation phenomenon, the
wave propagation model, developed for the flat specimens, has been identically applied to a cylindrical cable geometry. Therefore, despite the model that was developed for a PEA cell testing flat specimen, it works correctly also when the specimen has cylindrical geometry, just as the cable. This is because the presence of surface charges has been considered and, in particular, only those with acoustic waves propagating in the perpendicular direction.
In Figure 5, the configuration implemented into the model is schematically represented. It shows the section of the full-size cable and some parts of the PEA cell (of course, the entire PEA cell, as that of Figure 3, has been implemented into the model). In the same figure, the path of the waves 𝑝 (𝑡), 𝑝 (𝑡), and 𝑝 (𝑡) are also depicted. The detailed explanation of these acoustic waves course is given in the following.
Figure 5. Section of the cable under test and the part of the PEA cell, the latter being responsible of acoustic waves reflection. The paths of the positive and negative waves, both direct and reflected, are reported.
In Table 1, the main features of the PEA cell components and cable layers involved in the acoustic waves propagation, are listed.
Table 1. Main PEA cell and cable layers features.
Component Layer Material Sound Speed (m/s) Thickness (m)
Cable Dielectric XLPE 2200 20 × 10−3
Semiconductor Semiconductor ~2200 2 × 10−3
PEA cell Ground Electrode Aluminum 6420 30 × 10−3
Piezoelectric PVDF 2040 40 × 10−6
By implementing the values of Table 1 into the model and considering a pulse voltage of 8 kV, another behavior can be observed as shown by the simulation result in Figure 6.
As it can be seen from the figure below, the first acoustic wave 𝑝 (𝑡) is detected by the piezoelectric sensor at 4.66 μs, which is the time needed by the wave to travel within the ground electrode. The acoustic wave 𝑝 (𝑡), instead, needs around 14 μs (time useful to cross both the dielectric layer and the ground electrode) to reach the sensor. At the same time, due to the ground electrode thickness as well as to the cable dielectric layer features, reflected wave 𝑝 (𝑡) is sensed by
Figure 5. Section of the cable under test and the part of the PEA cell, the latter being responsible ofacoustic waves reflection. The paths of the positive and negative waves, both direct and reflected,are reported.
In Table 1, the main features of the PEA cell components and cable layers involved in the acousticwaves propagation, are listed.
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Table 1. Main PEA cell and cable layers features.
Component Layer Material Sound Speed (m/s) Thickness (m)
CableDielectric XLPE 2200 20 × 10−3
Semiconductor Semiconductor ~2200 2 × 10−3
PEA cellGround Electrode Aluminum 6420 30 × 10−3
Piezoelectric PVDF 2040 40 × 10−6
By implementing the values of Table 1 into the model and considering a pulse voltage of 8 kV,another behavior can be observed as shown by the simulation result in Figure 6.
As it can be seen from the figure below, the first acoustic wave p−(t) is detected by the piezoelectricsensor at 4.66 µs, which is the time needed by the wave to travel within the ground electrode.The acoustic wave p+(t), instead, needs around 14 µs (time useful to cross both the dielectric layer andthe ground electrode) to reach the sensor. At the same time, due to the ground electrode thickness aswell as to the cable dielectric layer features, reflected wave pR
−(t) is sensed by the sensor. In details,
by observing the previous Figure 5, after the wave p−(t) reaches the sensor, the latter is reflected in theopposite direction and, in correspondence of the ground electrode/cable interface it is reflected againin the sensor’s direction. Therefore, after (3 × 4.66) µs ≈ 14 µs the wave pR
−(t) reaches the sensor and
overlaps with the main positive signal p+(t).
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the sensor. In details, by observing the previous Figure 5, after the wave 𝑝 (𝑡) reaches the sensor, the latter is reflected in the opposite direction and, in correspondence of the ground electrode/cable interface it is reflected again in the sensor’s direction. Therefore, after (3 × 4.66) μs ≈ 14 μs the wave 𝑝 (𝑡) reaches the sensor and overlaps with the main positive signal 𝑝 (𝑡).
Figure 6. Simulation result for dielectric thickness equal to 20 mm and ground electrode thickness equal to 30 mm. The positive acoustic wave overlaps with the negative wave reflected within the ground electrode.
For a better visualization of the acoustic waves behavior around t = 14 μs, in Figure 7, a zoomed image is reported.
Figure 7. Zoom of simulation result in correspondence of the time in which the negative reflected wave and positive wave are superimposed.
This drawback occurs because the features of the cable under test (thickness and sound velocity of the dielectric layer) are not compatible with the employed PEA cell (with 30 mm thick ground electrode). In fact, if the same PEA cell is used, the maximum allowed XLPE dielectric thickness must be lower than 20 mm, as can be calculated by means of Equation (1). 𝑑 < 2𝑣 𝑑𝑣 ; (1)
where 𝑑 and 𝑑 are the thicknesses of the dielectric layer and the ground electrode, respectively. While, 𝑣 and 𝑣 are the sound velocities of the materials involved, such as dielectric and aluminum. To better explain how Equation (1) is derived, Figure 5 can be observed. Calling 𝑡 the time needed for an acoustic wave to cross the cable insulation layer, calculated as 𝑡 = 𝑑 𝑣⁄ , and 𝑡 the time for an acoustic wave to cross the ground electrode, 𝑡 = 𝑑 𝑣⁄ . By neglecting the semiconductor layer, it is possible to prevent the reflected acoustic wave 𝑝 (𝑡) from overlapping to the main acoustic wave
Figure 6. Simulation result for dielectric thickness equal to 20 mm and ground electrode thicknessequal to 30 mm. The positive acoustic wave overlaps with the negative wave reflected within theground electrode.
For a better visualization of the acoustic waves behavior around t = 14 µs, in Figure 7, a zoomedimage is reported.
Energies 2019, 12, x FOR PEER REVIEW 8 of 12
the sensor. In details, by observing the previous Figure 5, after the wave 𝑝 (𝑡) reaches the sensor, the latter is reflected in the opposite direction and, in correspondence of the ground electrode/cable interface it is reflected again in the sensor’s direction. Therefore, after (3 × 4.66) μs ≈ 14 μs the wave 𝑝 (𝑡) reaches the sensor and overlaps with the main positive signal 𝑝 (𝑡).
Figure 6. Simulation result for dielectric thickness equal to 20 mm and ground electrode thickness equal to 30 mm. The positive acoustic wave overlaps with the negative wave reflected within the ground electrode.
For a better visualization of the acoustic waves behavior around t = 14 μs, in Figure 7, a zoomed image is reported.
Figure 7. Zoom of simulation result in correspondence of the time in which the negative reflected wave and positive wave are superimposed.
This drawback occurs because the features of the cable under test (thickness and sound velocity of the dielectric layer) are not compatible with the employed PEA cell (with 30 mm thick ground electrode). In fact, if the same PEA cell is used, the maximum allowed XLPE dielectric thickness must be lower than 20 mm, as can be calculated by means of Equation (1). 𝑑 < 2𝑣 𝑑𝑣 ; (1)
where 𝑑 and 𝑑 are the thicknesses of the dielectric layer and the ground electrode, respectively. While, 𝑣 and 𝑣 are the sound velocities of the materials involved, such as dielectric and aluminum. To better explain how Equation (1) is derived, Figure 5 can be observed. Calling 𝑡 the time needed for an acoustic wave to cross the cable insulation layer, calculated as 𝑡 = 𝑑 𝑣⁄ , and 𝑡 the time for an acoustic wave to cross the ground electrode, 𝑡 = 𝑑 𝑣⁄ . By neglecting the semiconductor layer, it is possible to prevent the reflected acoustic wave 𝑝 (𝑡) from overlapping to the main acoustic wave
Figure 7. Zoom of simulation result in correspondence of the time in which the negative reflected waveand positive wave are superimposed.
Energies 2019, 12, 4186 9 of 13
This drawback occurs because the features of the cable under test (thickness and sound velocityof the dielectric layer) are not compatible with the employed PEA cell (with 30 mm thick groundelectrode). In fact, if the same PEA cell is used, the maximum allowed XLPE dielectric thickness mustbe lower than 20 mm, as can be calculated by means of Equation (1).
dd < 2vddGRvAL
; (1)
where dd and dGR are the thicknesses of the dielectric layer and the ground electrode, respectively.While, vd and vAL are the sound velocities of the materials involved, such as dielectric and aluminum.
To better explain how Equation (1) is derived, Figure 5 can be observed. Calling td the time neededfor an acoustic wave to cross the cable insulation layer, calculated as td = dd/vd, and tGR the time foran acoustic wave to cross the ground electrode, tGR = dGR/vGR. By neglecting the semiconductor layer,it is possible to prevent the reflected acoustic wave pR
−(t) from overlapping to the main acoustic wave
p+(t). The former indeed reaches the sensor after 3tGR, while the latter needs a time interval equal totd + tGR to reach the sensor and, as a result, the relationship 3tGR > td + tGR must be satisfied.
To facilitate and make a faster evaluation of the maximum dielectric thickness that can be testedwith the PEA cell having the ground electrode of 30 mm, a graph has been plotted in Figure 8.
Energies 2019, 12, x FOR PEER REVIEW 9 of 12
𝑝 (𝑡). The former indeed reaches the sensor after 3𝑡 , while the latter needs a time interval equal to 𝑡 + 𝑡 to reach the sensor and, as a result, the relationship 3𝑡 > 𝑡 + 𝑡 must be satisfied. To facilitate and make a faster evaluation of the maximum dielectric thickness that can be tested
with the PEA cell having the ground electrode of 30 mm, a graph has been plotted in Figure 8.
Figure 8. Graph useful to determine the maximum sample thickness that can be tested without reflections in the main output signal, on the basis of the 30 mm thick ground electrode.
Of course, though, dielectric thickness depends on the cable’s operating voltage. This means that the PEA cell must be adapted to the specific cable in accordance with the needs of the manufacturing company. However, to overcome the above discussed problem, and therefore to avoid the presence of the reflected wave 𝑝 (𝑡) in the main signal, the ground electrode of the PEA cell must be correctly sized and selected. For this aim, Equation (2), which is derived from Equation (1), can be used. 𝑑 > 12 𝑑 𝑣𝑣 (2)
As in the previous case, an abacus (see Figure 9) can be useful to easily determine the ground thickness on the basis of the cable features (thickness and sound velocity of the dielectric layer). In this case, in which the space charge must be measured in a cable with dielectric thickness equal to 20 mm and sound velocity of the XLPE equal to 2200 m/s, the correct ground electrode thickness must be greater than 34 mm.
Figure 9. Abacus for the choice of the minimum ground electrode thickness needed to avoid reflections in the main output signal. The red dotted line highlights our case in which the dielectric layer made of XLPE material have sound velocity of 2200 m/s and thickness 20 mm.
To verify the above-mentioned, a simulation has been made by changing the thickness of the ground electrode from 30 (see Figure 6) to 34 mm. As it can be seen from Figure 10, which is the output of the PEA cell model, the reflected wave 𝑝 (𝑡) is detected by the sensor at almost 16 μs, while the positive peak 𝑝 (𝑡) at a time equal to 14.6 μs.
Figure 8. Graph useful to determine the maximum sample thickness that can be tested withoutreflections in the main output signal, on the basis of the 30 mm thick ground electrode.
Of course, though, dielectric thickness depends on the cable’s operating voltage. This means thatthe PEA cell must be adapted to the specific cable in accordance with the needs of the manufacturingcompany. However, to overcome the above discussed problem, and therefore to avoid the presenceof the reflected wave pR
−(t) in the main signal, the ground electrode of the PEA cell must be correctly
sized and selected. For this aim, Equation (2), which is derived from Equation (1), can be used.
dGR >12
ddvALvd
(2)
As in the previous case, an abacus (see Figure 9) can be useful to easily determine the groundthickness on the basis of the cable features (thickness and sound velocity of the dielectric layer). In thiscase, in which the space charge must be measured in a cable with dielectric thickness equal to 20 mmand sound velocity of the XLPE equal to 2200 m/s, the correct ground electrode thickness must begreater than 34 mm.
Energies 2019, 12, 4186 10 of 13
Energies 2019, 12, x FOR PEER REVIEW 9 of 12
𝑝 (𝑡). The former indeed reaches the sensor after 3𝑡 , while the latter needs a time interval equal to 𝑡 + 𝑡 to reach the sensor and, as a result, the relationship 3𝑡 > 𝑡 + 𝑡 must be satisfied. To facilitate and make a faster evaluation of the maximum dielectric thickness that can be tested
with the PEA cell having the ground electrode of 30 mm, a graph has been plotted in Figure 8.
Figure 8. Graph useful to determine the maximum sample thickness that can be tested without reflections in the main output signal, on the basis of the 30 mm thick ground electrode.
Of course, though, dielectric thickness depends on the cable’s operating voltage. This means that the PEA cell must be adapted to the specific cable in accordance with the needs of the manufacturing company. However, to overcome the above discussed problem, and therefore to avoid the presence of the reflected wave 𝑝 (𝑡) in the main signal, the ground electrode of the PEA cell must be correctly sized and selected. For this aim, Equation (2), which is derived from Equation (1), can be used. 𝑑 > 12 𝑑 𝑣𝑣 (2)
As in the previous case, an abacus (see Figure 9) can be useful to easily determine the ground thickness on the basis of the cable features (thickness and sound velocity of the dielectric layer). In this case, in which the space charge must be measured in a cable with dielectric thickness equal to 20 mm and sound velocity of the XLPE equal to 2200 m/s, the correct ground electrode thickness must be greater than 34 mm.
Figure 9. Abacus for the choice of the minimum ground electrode thickness needed to avoid reflections in the main output signal. The red dotted line highlights our case in which the dielectric layer made of XLPE material have sound velocity of 2200 m/s and thickness 20 mm.
To verify the above-mentioned, a simulation has been made by changing the thickness of the ground electrode from 30 (see Figure 6) to 34 mm. As it can be seen from Figure 10, which is the output of the PEA cell model, the reflected wave 𝑝 (𝑡) is detected by the sensor at almost 16 μs, while the positive peak 𝑝 (𝑡) at a time equal to 14.6 μs.
Figure 9. Abacus for the choice of the minimum ground electrode thickness needed to avoid reflectionsin the main output signal. The red dotted line highlights our case in which the dielectric layer made ofXLPE material have sound velocity of 2200 m/s and thickness 20 mm.
To verify the above-mentioned, a simulation has been made by changing the thickness of theground electrode from 30 (see Figure 6) to 34 mm. As it can be seen from Figure 10, which is the outputof the PEA cell model, the reflected wave pR
−(t) is detected by the sensor at almost 16 µs, while the
positive peak p+(t) at a time equal to 14.6 µs.
Energies 2019, 12, x FOR PEER REVIEW 10 of 12
In this work, the pulse generator has been applied into the partially stripped outer semiconductive layer of the cable. In future work, in order to verify the effect of injecting voltage pulses into a coaxial cable, further two test methods will be considered, such as pulse injection into the cable conductor and pulse injection into the measuring electrode [35].
Figure 10. Simulation result for dielectric thickness equal to 20 mm and ground electrode thickness equal to 34 mm. The negative reflected wave falls outside the positive acoustic wave.
8. Conclusion
The purpose of this work was to provide useful information to cable manufacturing industries in order to avoid incorrect evaluation of the space charge profiles measured with the PEA method. In addition, a complete summary of the protocol to be followed and suggested by the IEEE Std 1732-2017 has also been given.
In this aim, an experimental test has been carried out inside a high voltage laboratory of a cables industry supporting the research. Measurement result obtained for a full-size XLPE insulated cable shows that the space charge profile is not clear and therefore its interpretation turns to be complex.
The obtained experimental results prove to be quite useful for cable industries that carry out pre-qualification and type tests and thus need to satisfy the standard IEEE 1732-2017. The latter standard calls for the performance of space charge measurement in cable specimens. The advantages provided by this study are useful for the choice of the PEA cell features (in particular the thickness of the ground electrode thickness) on the basis of the manufactured cables (thickness and sound velocity of the dielectric layer). Therefore, the suggestions reported in this work may be used to obtain a clear PEA cell output signal (without noise signals within the main charge profile) and thus avoid its mis-interpretation. Beyond this, a clear space charge profile allows to obtain a clear electric field distribution, which, as requested by the standard IEEE 1732-2017, must be carefully analyzed. Moreover, the model results provide a very useful tool in order to differentiate the main signal from the reflections.
The solution to the issues encountered in the proposed work has been given after analyzing simulation results provided by a PEA cell model. Simulation results show that the overlap of the acoustic waves was essentially due to the ground electrode thickness. In fact, after changing the ground electrode dimensions from 30 to 34 mm the negative reflected wave falls over the main positive space charge peak. In this way, at least in simulation, the PEA cell output signal is clear and easy to be correctly interpreted. To facilitate the choice of the ground electrode thickness as well as the maximum dielectric features that can be tested for a given PEA cell, two useful graphs are also provided.
In conclusion, it is important to notice that in the simulation model the attenuation of the acoustic waves has not been taken into account. This can be made in future work, as well as a further experimental test with the greater ground electrode thickness.
Author Contributions: Conceptualization, A.I. and P.R.; methodology, A.I. and G.R.; software, A.I.; validation, P.R. and F.V.; formal analysis, A.I., P.R. and G.R.; investigation, A.I.; resources, A.I.; writing—original draft preparation, A.I.; writing—review and editing, A.I., P.R. and E.R.S.; supervision, G.C. and G.A.
Figure 10. Simulation result for dielectric thickness equal to 20 mm and ground electrode thicknessequal to 34 mm. The negative reflected wave falls outside the positive acoustic wave.
In this work, the pulse generator has been applied into the partially stripped outer semiconductivelayer of the cable. In future work, in order to verify the effect of injecting voltage pulses into a coaxialcable, further two test methods will be considered, such as pulse injection into the cable conductor andpulse injection into the measuring electrode [35].
8. Conclusions
The purpose of this work was to provide useful information to cable manufacturing industriesin order to avoid incorrect evaluation of the space charge profiles measured with the PEA method.In addition, a complete summary of the protocol to be followed and suggested by the IEEE Std1732-2017 has also been given.
In this aim, an experimental test has been carried out inside a high voltage laboratory of a cablesindustry supporting the research. Measurement result obtained for a full-size XLPE insulated cableshows that the space charge profile is not clear and therefore its interpretation turns to be complex.
The obtained experimental results prove to be quite useful for cable industries that carry outpre-qualification and type tests and thus need to satisfy the standard IEEE 1732-2017. The latterstandard calls for the performance of space charge measurement in cable specimens. The advantages
Energies 2019, 12, 4186 11 of 13
provided by this study are useful for the choice of the PEA cell features (in particular the thicknessof the ground electrode thickness) on the basis of the manufactured cables (thickness and soundvelocity of the dielectric layer). Therefore, the suggestions reported in this work may be used toobtain a clear PEA cell output signal (without noise signals within the main charge profile) and thusavoid its mis-interpretation. Beyond this, a clear space charge profile allows to obtain a clear electricfield distribution, which, as requested by the standard IEEE 1732-2017, must be carefully analyzed.Moreover, the model results provide a very useful tool in order to differentiate the main signal fromthe reflections.
The solution to the issues encountered in the proposed work has been given after analyzingsimulation results provided by a PEA cell model. Simulation results show that the overlap of theacoustic waves was essentially due to the ground electrode thickness. In fact, after changing theground electrode dimensions from 30 to 34 mm the negative reflected wave falls over the main positivespace charge peak. In this way, at least in simulation, the PEA cell output signal is clear and easy to becorrectly interpreted. To facilitate the choice of the ground electrode thickness as well as the maximumdielectric features that can be tested for a given PEA cell, two useful graphs are also provided.
In conclusion, it is important to notice that in the simulation model the attenuation of the acousticwaves has not been taken into account. This can be made in future work, as well as a furtherexperimental test with the greater ground electrode thickness.
Author Contributions: Conceptualization, A.I. and P.R.; methodology, A.I. and G.R.; software, A.I.; validation, P.R.and F.V.; formal analysis, A.I., P.R. and G.R.; investigation, A.I.; resources, A.I.; writing—original draft preparation,A.I.; writing—review and editing, A.I., P.R. and E.R.S.; supervision, G.C. and G.A.
Funding: This research was funded by Prysmian Group S.p.A.
Acknowledgments: The authors thank Luca De Rai, Stefano Franchi Bononi and Marco Albertini, PrysmianGroup S.p.A., for the support in the measurements at the high voltage laboratory in Milan.
Conflicts of Interest: The authors declare no conflict of interest.
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